LCGB Forums

Hi all, need some advise , friends engine piston holed
From ? Half throtel .
Changed timing set at 16 : now runs better but still pinking but now before half throtel .
Confused .com.
Fuel is premium 3% fully.
Can't figure this one out .

air leak, wrong jetting......

what cylinder, carb' (with settings), exhaust, etc....

Muggello 186 mikuni 28 tzr evo .
Would jetting change with the timing.?

Jimmyjames wrote:Muggello 186 mikuni 28 tzr evo .
Would jetting change with the timing.?

Why did it Hole the Piston was it Running To lean was Timing To Advanced , Air Leak ? The Carb Jetting Wont change with the Timing but people will need to know this to give you some ideas /Way Forward

Jimmyjames wrote:Muggello 186 mikuni 28 tzr evo .
Would jetting change with the timing.?

The way forward is to set your timing 17/16 degree then adjust your jetting to suit.

My monies on an air leak. Take the carb' and exhaust off, leaving carb' manifold and exhaust stub. Get a mountain bike inner tube and cut it in half. Connect one half to manifold and the other to the stub. Pump up to no more than 5 psi and squirt soapy water around suspect areas........watch for bubbles. Put a pressure gauge on the tyre valve and watch for a drop in pressure. If no bubbles and pressure hold there's no leak......if you get bubbles and a drop in pressure you've found the likely cause of the pinking.

The holed piston I agree most likely an air leak .
I've changed timing to 16 /.
But still pinking.

Is the TDC timing mark correct to start with ?

Has it been strobed to see if ignition timing wanders?

Is it pinking or other mechanical noise ?

Top dead center could have changed, he put barrel on with out a base gasket so this will mean we need to check again.
Tdc I mean.
Would a auto retard box cure the pinking?

I've seen a huge range of compression ratios used with Mugellos, some of which are way too high (probably intended for Imola use), others that are really low and kill performance (probably a knee-jerk reaction to the overly high comp heads sometimes used), others that don't seem to suit any and some that will suit an early Mugello but not a late one and vice versa. Unfortunately, to address this issue you would need to confirm the version of Mugello kit you have, the state of tune (virgin or reworked), and the measure the volume of the head so that you can determine the compression ratio. NB: This is an issue of 'corrected' compression ratio, determined in respect of port durations, rather than the geometric compression ratio.

NB: If the corrected compression ratio is way too high for your barrel then you will need to rework the head or source another. Unfortunately, this may not be the case so you cannot simply fit a lower compression head as that would kill performance if compression is not the problem in this case. The only way to know for sure is to remove the head and measure the volume... and determine state of barrel tune.

Best of luck with it.

Adam

I use auto retard but not to mask problems. Mine retards to16 where you currently are.
If I was looking at this I would remark TDC and use a strobe as a basic minimum.

Reading through the whole thread, you’ve received excellent advice to which I’d like to add my own thoughts.

Firstly, TDC will not change regardless of base packing, head gaskets etc.

The other concern that I have experienced (before I knew better!) was running too tight a squish, in which case the piston will be hitting the head. This might just be a little (which is still way too much) but it sounds just like pinking. You should check the squish in any case. Head misalignment will also allow the head to come into contact with the piston but I suspect the head is spigotted to the barrel to centralise it by design.

As Adam says, the head volume should also be measured as this is so often overlooked & an essential element in a successful engine build, regardless of whether the principles of geometric compression ratios or corrected are applied. If you were to report back on here the measured volume, Forum Members can then advise further...

A lot of info guys thanks ,
I think this is going to be a long slog
Let's hope we make it to Adria.
Lots of spares

I'd look to resolve it before leaving

Adam

+ 1

forget the spares......just get European Recovery...

Adam_Winstone wrote:.....................I've seen a huge range of compression ratios used with Mugellos, some of which are way too high (probably intended for Imola use),

I bought a cylinder head for my Mk1 Mugello many years ago which was engraved by 'Reaper' and said Mugello/Imola no wonder it holed 2 pistons

Yep, MK1 had very low exhaust port duration as standard so the last thing that you wanted to do was bolt a skimmed head to it, resulting in very high corrected comp ratio and... a tendency to hole pistons.

Guess how I learnt?

Adam

Strange , my mugello kit has never given me any problems ,
Rides smooth and true.
We will have to ditch the kit and go for something else.
Must be a bad egg.

Good kits so try to resolve it before ditching it. You may buy similar issues with any kit that you choose as the replacement!

My early 225 mugelo had comp ratio too high , ran great to start with then the heat used to just grow and the pinking would start , I reduced the ratio and its still running 12 years later same piston and no replates .